The Angel Building, a 260,000sqft office development by Derwent in Clerkenwell, in central London, was completed in 2010. In spite of a poor economic climate, two thirds of the building was let before completion, and full occupancy was reached very soon after opening. For Johanna Gibbons, landscape architect for the project, “there is no doubt that the environmental quality afforded through the successful retention of mature trees all around the building was a key differentiator for this infill development. A tree constraints plan was developed early and became a primary driver for the design. The curved building façade on St John Street was positioned and shaped in response to the location of the root protection area. The strategy was to retain groups of trees, primarily limes (Tillia spp.) and Caucasian wingnuts (Pterocarya fraxinifolia), at the corner of the project so as to create a microclimate facilitating building ventilation and energy conservation as well as the establishment of new tree and soft landscape around the scheme.

Spatial constraints were such that the access road for construction had to encroach upon the root protection area and deliveries needed to be conducted on a daily basis. To enable this, the Tree Protection Plan (TPP) and Arboricultural Method Statement (AMS), developed by appointed tree specialist JCA, in coordination with the project landscape architect and the council tree officer, proposed the use of a porous load-spreading cellular confinement system (Geoweb) braced with timber frames. Waiting areas for delivery trucks were marked, and equipped with oil trays to avoid leakage of any potential pollutants into the ground. The 80-ton crane used for construction was set outside the root protection area, on a temporary concrete raft, which also helped to avoid compaction and contamination of the soils below. The trees to be retained were pruned prior to construction starting to avoid branches being inadvertently clipped by machinery and to help the trees conserve energy. All existing trees were irrigated during the two-year construction period following a sporadic pattern imitating rain. Because irrigation was fed with calcium- rich London tap water, the system was fitted with filters to avoid increasing the soil pH. Once the temporary access road was removed, and prior to final surfacing or landscaping being installed, earthworms were used to aerate the soils and help mix-in imported organic matter, without causing any disruption to tree roots.

Philip Wood, then tree officer for the council, emphasises that “monitoring, and monitoring of the monitoring” were critical to the project success. Soil compaction testing using a penetrometer was conducted before works started, mid-way through and after project completion to check that ground protection measures were effective. As Philip explains “this showed that no significant changes had occurred in soil compaction in the RPA where the council had allowed for incursions to occur – but if deterioration had been detected, this approach would have enabled us to take corrective action”. Further to this, the TPP required the tree consultant to submit monthly reports with photographs. Council officers had allocated time to review the reports, ask questions when needed, and conduct some site visits. This not only ensured the TPP was thoroughly adhered to, but also allowed for day-to-day issues to find easy (tree friendly) resolutions.​The new London plane trees (Platanus x hispanica) planted in front of the building entrance were ordered well in advance and hand-picked from the supplying nursery. To ensure good integration with existing bus traffic on St John Street, the specification required that a five-metre clear stem be achieved at least 18 months prior to planting. Below-ground conditions were optimised by integrating arboricultural advice prior to locating the main utility corridor – so as to retain an adequate rooting area for the new trees.